Natural and synthetic fibers are typically formed by drawing a large number of individual filaments from a plurality of apertures or orifices generally located in the bottom of a reservoir containing the molten material to be fiberized. In the case of polymers such as nylon, rayon, etc., these orifices are referred to spinnerets. In the case of glass fibers, these orifices are generally known as tips.
Strand comprised of glass fibers may be formed by attenuating single filaments from as many as 4,000 individual tips located on the bottom of a fiber glass bushing assembly. The individual filaments are usually coated with a chemical binder composition and then gathered into one or more strands before being subsequently wound onto one or more tubes held centrifugally on a rotating collet in order to produce what are known as forming packages. The process is carefully monitored to maintain uniform filament diameters during the high speed attenuation process.
Fiber glass bushing assemblies are typically constructed from precious metals such as platinum and platinum-rhodium alloys. Grain stabilized platinum or alloys thereof may also be employed. An alloy having 80 percent platinum and 20 percent rhodium by weight is commonly used. This same alloy is used for the construction of all other bushing components that come in direct contact with molten glass, such as the tips, sidewalls, etc. Generally, any surface in direct contact with molten glass is usually made from a platinum-rhodium alloy or some other alloy of precious metal.
The bushing itself is electrically heated by means of an electrical current applied through suitable connectors to an electrical terminal located on each of two opposing sides of the bushing. This arrangement is well understood by those skilled in the art and is fully described in "The Manufacturing Technology of Continuous Glass Fibers", by K. L. Loewenstein, Elsevier Publishing Company, 1973, at pages 110-114.
Strand formed from individual fiber glass filaments may be used for many purposes. For example, the strand may be unwound from finished forming packages and then twisted by means of a twist frame onto a bobbin in order to form a yarn for subsequent use in textile products. Some of this yarn may be rewound from bobbins and onto warp beams for use in the production of woven fabrics. Equal lengths of yarn must be wound carefully onto the beams at precisely controlled tensions in order to ensure a high-quality fabric. Other applications include the use of continuous and chopped glass strand to reinforce plastic resins. A large number of applications for such reinforced resins can be found in the automotive, housing, marine and aircraft industries and particularly in the electronic industry for the production of printed circuit boards.
In many of the applications discussed above, it is desirable that the diameter or cross section of the individual filaments produced during the manufacturing process be as uniform as possible. One is primarily interested in controlling the production process so as to maintain a uniform diameter or cross-sectional area along the entire length of each filament. If the cross-sectional area is multiplied by the density of the glass, the reciprocal of the result is a number usually expressed in units of hundreds of yards per pound. This measure has come to be referred to in the art as the "yardage" of the particular strand. Since glass strand is often sold in pound quantities, a customer purchasing 500 pounds of a particular strand would expect to realize a predictable number of yards. This unit of measure is very important in textile applications where what is known as "G-75 yarn" is sold on bobbins and one would expect that such a bobbin weighing 10 pounds would nominally contain 75,000 yards of strand.
Variations in individual filament diameter can ultimately have a disastrous impact upon finished product quality. For example, even though the variation in an individual filament may be negligible, when the variations of several hundred filaments are combined into a single strand, the individual imperfections are magnified. When fiber glass strands are used to reinforce a product such as a printed circuit board, in which there is a great need for dimensional uniformity, the use of strand having an unpredictably varying diameter will result in a board having poor physical properties and dimensional tolerances.
In the process of fiberizing or attenuating a molten material such as glass into individual filaments, not one but several parameters must be carefully controlled. Taken separately, there are as many as 10 to 20 variables in the overall process, such as the temperature of the tip, the temperature of the molten glass inside the bushing, the dependence of the viscosity upon temperature, the flow rate through the tip, and the drawing force or tension required to attenuate the fiber. All of these variables tend to interact with one another as a coupled non-linear system. Small changes in one variable, such as temperature for example, may produce a large effect upon glass viscosity, the overall drawing force and the filament diameter or yardage. Also, since the system is dynamic, it is possible that certain unique combinations of process variables may result in the generation of self-sustained oscillations in the diameter of the filament. The avoidance of these oscillations is the primary motivation for controlling the yardage variation problem.
One of the most influential parameters in the glass fiber-forming process is that of the tip temperature. It is well known by those skilled in the art that the viscosity of glass is highly dependent upon temperature. It is also well understood by those skilled in the art that the flow rate of glass through a tip is inversely proportional to the viscosity according to the Hagen-Poiseuille law which governs the flow of fluids inside circular pipes or ducts. If it is assumed that the measurement of tip temperature also represents a more or less good indication of the temperature of the molten glass flowing through the tip, then it is possible to control the flow rate of the glass through the tip by means of a closed-loop feed back system. Filament diameter and cross-sectional area can be statistically correlated as a function of tip temperature. For example, if the cross-sectional area of a filament or strand comprised of such filaments tended to decrease over a period of time, indicating that at a fixed take-up speed the net flow rate was decreasing so as to result in finer and finer filaments, then a bushing controller could be signaled by means of such a feedback system to increase the electrical current flow to the bushing in order to increase the temperature of the tip and thus reduce the viscosity of the glass thereby increasing the flow rate back to a level where the production of a filament having the required diameter will be reestablished.
Thus, there is a need to control the cross-sectional area and reduce the yardage variation associated with the production of individual fiber glass filaments and strand. There is also a need to accurately monitor, in real time, the diameter variation of a fiber glass strand. There also exists a need to automatically compensate the bushing tip temperature based on the variation in yardage so as to stabilize the process and produce strand having a uniform cross-sectional area.
Laser diffraction methods for measuring the diameter of cylindrical objects have developed rapidly in the last few years, primarily in response to the strict process control requirements of high quality optical fibers. Due to their high degree of internal reflection and refraction, the use of large angle forward scattering and diffraction methods for the diameter measurement of optical fibers is possible because a large amplitude signal is transmitted and therefore available for analysis. Fiber glass strands are composed of a multiplicity of individual filaments that are only diffusely reflecting and practically opaque thus precluding the use of large angle forward scattering methods. The use of a conventional profile or shadow-type imaging system employing CCD detectors to measure strand diameter is also generally precluded due to practical considerations. For example, a typical strand is about 200 microns in diameter and moves in excess of 150 feet per second. In a CCD detector having discrete pixels or photo-sites located on roughly 13 micron centers, a unit magnification would produce an image only 15 pixels across. The use of higher magnification is a possible solution to this problem but it would either require an impractically long optical system or the use of prohibitively expensive folded optical paths. Additionally, the dimensional measurements made with pixels located on 13 micron centers can at best be mediocre in the absence of a good subpixel interpolation scheme especially where measurements of submicron accuracy may be required. Subpixel interpolation is usually done by a "gray level" method, i.e., a gray registering pixel is straddled by a white and black registering pixel and the location of a solid edge is then linearly interpolated based on the gray-level intensity.
The instant invention overcomes the practical disadvantages associated with the use of conventional profile or shadow-type imaging systems and the impossibility of using large angle forward scattering techniques to measure the diameter variation of a nearly opaque moving glass strand by providing both an apparatus and several methods for the analysis of the Fresnel diffraction pattern that results when the moving strand is interposed between a beam of electromagnetic radiation emitted from a pulsed semiconductor diode laser and directed at the sensing surface of a CCD detector.